1. Field of Invention
The present invention relates to a photomask and a patterning method using the photomask. More particularly, the present invention relates to a half-tone phase shift mask and a patterning method using the half-tone phase shift mask.
2. Description of Related Art
Following the increase in the level of circuit integration in semiconductor devices, the accuracy of each semiconductor fabrication process is increasingly important because minor errors might result in batch failures and tremendous production lost.
Photolithography is one of the most important processes for forming a semiconductor device. In fact, most etching or doping processes must be preceded by a photolithographic operation. In photolithography, resolution and depth of focus (DOF) are often the two most important factors in determining the final quality of a product.
In the manufacturing of semiconductor devices with a feature size greater than 0.18 μm, a binary mask of the type having a coated chromium pattern on a quartz plate is often good enough to transfer a high-quality pattern to a photoresist layer. However, as the feature size is reduced to a value lower than 18 μm (for example, 0.13 μm), light diffraction around holes and lines has become an increasing serious problem due to a reduction in hole diameter and line width. To prevent problems caused by the diffraction of light through small holes and thin lines, phase shift mask (PSM) technique is often employed to transfer a pattern on the mask to a photoresist layer. Furthermore, to fabricate feature lines with a reduced dimension, assistant bars, chromeless phase lithography (CPL), alternating phase shift mask (altPSM)(or Levenson PSM) together with a 248 nm wavelength light source are frequently used.
Among the aforementioned techniques, the alternating phase shift mask is a useful technique for increasing the resolution of some special areas (such as a true gate region or a region having a gate conductive layer crossing over an active region). However, to implement the alternating phase shift mask technique, the so-called double-exposure operation needs to be carried out. In other words, two different types of mask must be used to complete the photo-exposure operation. For example, to fabricate the gate pattern (thick end sections with a narrow middle section) as shown in FIG. 1, a binary mask 200 (the blank portions are transparent regions 202 and the area with slashed lines is a non-transparent region 204) as shown in FIG. 2A is used to carry out a first photo-exposure. Thereafter, a phase shift mask 210 (the blank portions are transparent regions 212 and the area with slashed lines is a non-transparent region) as shown in FIG. 2B is used to carry out a second photo-exposure. There is, however, one major drawback in using the double-exposure technique. Since two different masks are used, the probability of having an alignment error is increased and hence the yield may drop. On the other hand, if a reticle is set up on the mask or a specially designed program is incorporated to control the movement of a photo-exposure station (such as a stepper), one is faced with an increase in the production cost.